Re: [PATCH v2] sched/core: Fix reset-on-fork from RT with uclamp

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 16, 2020 at 01:03:16PM +0200, Dietmar Eggemann wrote:
> On 16.04.20 10:59, Quentin Perret wrote:
> > uclamp_fork() resets the uclamp values to their default when the
> > reset-on-fork flag is set. It also checks whether the task has a RT
> > policy, and sets its uclamp.min to 1024 accordingly. However, during
> > reset-on-fork, the task's policy is lowered to SCHED_NORMAL right after,
> > hence leading to an erroneous uclamp.min setting for the new task if it
> > was forked from RT.
> > 
> > Fix this by removing the unnecessary check on rt_task() in
> > uclamp_fork() as this doesn't make sense if the reset-on-fork flag is
> > set.
> > 
> > Fixes: 1a00d999971c ("sched/uclamp: Set default clamps for RT tasks")
> > Reported-by: Chitti Babu Theegala <ctheegal@codeaurora.org>
> > Reviewed-by: Patrick Bellasi <patrick.bellasi@matbug.net>
> > Signed-off-by: Quentin Perret <qperret@google.com>
> > ---
> > Changes in v2:
> >  - Added missing 'Fixes:' tag (Patrick)
> >  - Removed unnecessary local variable (Doug, Patrick)
> > ---
> >  kernel/sched/core.c | 9 ++-------
> >  1 file changed, 2 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 3a61a3b8eaa9..9a2fbf98fd6f 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1232,13 +1232,8 @@ static void uclamp_fork(struct task_struct *p)
> >  		return;
> >  
> >  	for_each_clamp_id(clamp_id) {
> > -		unsigned int clamp_value = uclamp_none(clamp_id);
> > -
> > -		/* By default, RT tasks always get 100% boost */
> > -		if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> > -			clamp_value = uclamp_none(UCLAMP_MAX);
> > -
> > -		uclamp_se_set(&p->uclamp_req[clamp_id], clamp_value, false);
> > +		uclamp_se_set(&p->uclamp_req[clamp_id],
> > +			      uclamp_none(clamp_id), false);
> >  	}
> >  }
> 
> LGTM.
> 
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

Thanks guys!